1. Field of the Invention
The present invention relates to an image capturing apparatus such as a digital camera or a video camera, and particularly relates to an image capturing apparatus that performs focus detection based on the output from an image sensor.
2. Description of the Related Art
Technology for eliminating the need for a dedicated AF sensor and realizing high-speed phase difference AF on an image sensing plane by providing a phase difference detection function to an image sensor has been proposed. For example, according to Japanese Patent Laid-Open No. 2000-156823, in part of light receiving elements (pixels) on an image sensor, a sensitive area of a light receiving unit is positioned eccentrically with respect to an optical axis of an on-chip microlens, thereby providing a pupil division function. These pixels are used as focus detection pixels and disposed among a group of image forming pixels at predetermined intervals in order to perform phase difference type focus detection. Here, there are differences in light receiving characteristics between the focus detection pixels and dedicated image forming pixels. For example, the focus detection pixels have a smaller light receiving unit area than the dedicated image forming pixels, or the position of the center of gravity of a light receiving area of the focus detection pixels is shifted from the optical axis of the on-chip microlens. Thus, at locations where the focus detection pixels are disposed, image information is partially lost, and image signals are therefore generated by performing an interpolation operation based on information about dedicated image forming pixels in the neighborhood of those focus detection pixels.
Here, if the focus detection pixels are disposed at a sparse density, image quality degradation due to the above-described pixel defect can be alleviated, but on the other hand, focus detection image sampling characteristics deteriorate, resulting in a decrease in focus detection performance. That is to say, in this technical field, the manner in which the focus detection pixels are disposed is important in order to achieve both maintenance of focus detection performance and prevention of image quality degradation.
According to a technique disclosed in Japanese Patent Laid-Open No. 2007-279312, a first focus detection pixel group and a second focus detection pixel group are provided, wherein the first focus detection pixel group having a short image detection pitch performs high-precision focus detection and the second focus detection pixel group whose image detection pitch has been increased by narrowing a light receiving area of focus detection pixels performs detection of a large defocus amount, thereby achieving both high-precision focus detection and detection of a large defocus amount.
However, the above-described known techniques suffer the following problems. According to the technique disclosed in Japanese Patent Laid-Open No. 2007-279312, first focus detection pixels are disposed densely in order to detect the focus status in the vicinity of an in-focus position with high precision. Also, image signals of the focus detection pixels are generated by interpolation processing using output signals of dedicated image forming pixels in the neighborhood of the focus detection pixels. However, in the case of receiving light from an object having a high spatial frequency and other cases, the continuity of an image in the position of a focus detection pixel is less than that of images in the positions of the dedicated image forming pixels in the neighborhood of that focus detection pixel. Accordingly, if an interpolation operation is performed based on information about the dedicated image forming pixels in the neighborhood of the focus detection pixel, there is a possibility that the images obtained by the neighboring pixels may be very different from output of an image forming pixel that originally should be located in the position of the focus detection pixel. Moreover, disposing the focus detection pixels densely imposes a limitation on neighboring image forming pixel options that can be referred to in performing an interpolation operation, and thus makes it highly probable that the correction error will increase.